Compound semiconductors have been noted as the most suitable semiconductors for obtaining semiconductor devices which can satisfy recent demands for super high-speed operation and high performance. In some compound semiconductors, semiconductor mixed crystals can be synthesized which passes the zinc-blende crystal structure, and it is possible to obtain heterojunctions at which lattice matching between various dissimilar semiconductor crystal is achieved. Therefore, by utilizing such heterojunctions, devices such as ultra-high-speed electronic devices utilizing modulated doping, various optical devices such as laser diodes and novel electronic devices utilizing the superlattice structure have been developed and some of them have been already used in practice.
Furthermore, novel devices utilizing the new physical properties of such heterojunction structures or superlattice structures and especially compound semiconductor devices typically represented by lasers and HEMT (high electron mobility transistors) have been recently extensively studied at high speed. Such elements comprise in general a substrate and a thin film which is formed on the substrate and have a complicated structure consisting of many layers of dissimilar compounds. And it is well known to those skilled in the art that the performance of these elements is greatly influenced by the structure of the thin film, especially the sharp distribution of composition or the sharp distribution of doping concentration at the interface between the adjacent layers.
It follows, therefore, that the most fundamental technique for fabricating the heterojunction structures or the superlattice structures is to grow the layers each in a desired thickness of different compounds in such a way that the interface between the adjacent layers can be defined as sharply as possible, without causing any crystal defect.
One process for growing a thin III-V compound semiconductor film over the surface of a substrate is molecular beam epitaxy (MBE).
In the case of growing a III-V compound crystal by the conventional MBE method, at least one element of Group III and at least one element of Group V of the periodic table are simultaneously supplied on the surface of a substrate which disposed in a vacuum chamber and heated. (Chang, L. L. et al.; J. Vac. Sci, Technol., Vol. 10, P. 11, 1973). In this case, in order to prevent the escape of the Group V element having a high vapor pressure from the grown layer, a large amount of the Group V element is normally supplied on the surface of the growing layer. As a result, the Group III element immediately combines with the Group V element on the growing surface so that III-V molecules are formed. In order to obtain a flat grown surface in each atomic layer, the molecules thus obtained must be sufficiently diffused over the grown surface. However, the diffusion coefficient of such molecules is extremely low as reported by Neave, J. H. et al. (Appl. Phys. Lett. Vol. 47, P. 100, 1985), so that small hills and small valleys which differ in height by a few atomic layers always exist on the grown surface as shown in FIGS. 1A-1C. In FIG. 1, reference numeral 1 represents a substrate; 2, Ga atoms; and 3, As atoms. The Ga-As molecules thus formed on the substrate 1 are gradually increased in number in the order of steps shown in FIGS. 1A, 1B and 1C, but as shown in FIG. 1B, the second Ga-As molecular layer is formed before the first Ga-As molecular layer has completely covered the surface of the substrate 1. Since the mobility of Ga-As molecules is low, the second and third Ga-As layers are formed before the second Ga-As molecular layer is displaced to fill the openings of the first Ga-As layer, so that part of the second and third Ga-As molecular layers tends to be directly adsorbed in the openings of the first Ga-As layer and consequently small hills and valleys are formed as shown in FIG. 1C.
The results of recent studies on MBE show that when a RHEED (reflection high energy electron diffraction) pattern is observed when growing a thin film, the intensity of the light from the diffraction pattern varies periodically in time and that the period is equal to the length of time required for growing one Ga-As layer. FIG. 2 shows the mode of oscillation, the intensity of light from the diffraction pattern being plotted along the ordinate while is plotted, along the abscissa. It is readily understood that the oscillations of the intensity of light are damped in time. In the conventional MBE method, prior to the growth of a layer, As molecules are supplied over the surface of the substrate and as a result of the heat treatment for a long period of time, the surface of the substrate becomes relatively flat in terms of an atomic layer. The intensity of RHEED before the growth of a film, that is, t.ltoreq.0 represents the flatness of the surface of the substrate. When the supply of Ga atoms is started in addition to that of As molecules (that is, t&gt;0), the intensity of RHEED is rapidly reduced and then becomes a minimum level. The intensity of RHEED reflects the interference effect due to the surface roughness and the fact that the absolute value of the intensity of RHEED drops means that the ratio of the flat areas of the upper layers to those of the adjacent lower layers formed by respective steps becomes smaller. On the other hand, the reduction in amplitude of oscillations is a result of the increase in number of steps so that eventually only white noise signals are obtained. Therefore, the minimum level of RHEED means that one half of a single molecular layer has been grown and that the upper and lower surface areas of a step becomes substantially equal to each other. Thereafter, as the layer is grown further, the intensity of RHEED becomes a maximum level again. This means that just one molecular layer has been grown in a microscopic view.
It is very important to note the fact that the maximum value of each period is considerably lower than that of the preceding period. The reason is that since the mobility of the Ga-As molecules on the grown surface is not sufficient as described above with reference to FIGS. 1A-1C, a flat atomic surface cannot be reproduced after the growth of one molecular layer and steps of heights of one to a few atomic layers remain from point to point. The above-described tendency is further intensified as the growth of the film is increased, so that the oscillations of the intensity of RHEED disappear after the growth of tens of molecular layers as shown in FIG. 2. The reason is that many steps are formed on the grown surface as shown in FIG. 1C.
In the case of growing the thin film layers by the conventional MBE method, the grown surface is rough, that is, has little hills and valleys. As a result, when dissimilar semiconductor layers are grown over the grown surface so as to form a heterojunction interface, it cannot be avoided that the roughness of the interface of the heterojunction is intensified. Therefore, there has been proposed a method for accelerating the surface diffusion of molecules by interrupting the growth of the film and maintaining the grown layer at a high temperature (Sakaki, H. et al.; Japan. J. Appl. Phys. Vol. 24, L. 417, 1985). However, since the diffusion rate of the molecules is low inherently, the above-described method can improve the surface roughness to a small extent. Referring to FIGS. 3A-3C, the method for smoothing the surface of the grown layer by maintaining the surface at a high temperature according to the conventional process will be described in conjunction with the formation of the heterojunction by forming an AlAs layer over a GaAs layer. After a GaAs layer 4 has been grown to a predetermined thickness (FIG. 3A), the surface of the GaAs layer 4 is smoothed by thermally displacing "lands" of the thickness of one to three atomic layers on the surface of the GaAs layer 4 (FIG. 3B) and then the AlAs layer 5 is grown over the surface of the GaAs layer 4 (FIG. 3C).
However, according to the above-described process, the interface 6 of the heterojunction can be made flat within the range between a few .mu.m to 100 .mu.m, but in macroscopic view, the surface smoothness is increased as shown in FIG. 3C.
Because of the roughness of the interface of the heterojunction, devices such as quantum well semiconductor lasers and superlattice optical modulators which are constructed by various combinations of heterojunctions defined between the thin films are increased in breadth of spectrum and degraded in performance to a considerable extent. In order to improve the conventional MBE method, Japanese patent application Laid-Open No. 60-112692 discloses a method in which the Ga molecular beam and the As molecular beam are alternately irradiated on a GaAs substrate. However, according to this method between the As molecular beam irradiation period and the Ga molecular beam irradiation period, there is a period of time in which no molecule is irradiated, so that there is a danger that the As atoms escape from the grown crystal. Furthermore, it has the disadvantage that it takes a long period of time to grow a crystal.
Furthermore, the adverse effects caused by a low diffusion rate are further pronounced as the crystal growing temperature is lowered. For instance, it is impossible to obtain a high-quality epitaxially grown crystal of GaAs or InP at temperatures lower than 500.degree. C. and it is also impossible to obtain a high-quality AlAs epitaxial crystal at a growing temperature lower than 600.degree. C. Moreover, it becomes almost impossible to grow a GaAs or InP crystal at a temperature lower than 400.degree. C.and it also becomes almost impossible to grow an AlAs crystal at a temperature lower than 450.degree. C. As a result, the crystal growth is carried out at a high temperature so that diffusion of impurities results during the crystal growth. As a result, the distribution of impurities cannot be sharply defined and accordingly the characteristics of lasers, transistors and the like are limited to levels remarkably lower than their theoretical characteristics.
Another process for growing a thin III-V compound semiconductor film over the surface of a substrate is a metallo-organic chemical vapor deposition process (MOCVD). The same inventors improved MOCVD and proposed a method in which while a very small quantity of a hydride of an element of Group V is normally made to flow over the surface of a heated substrate together with a carrier gas, a metallo-organic compound containing an element of Group III and a hydride of an element of Group V which is diluted to a high concentration are alternately made to flow over the surface of the substrate together with the carrier gas, whereby a III-V compound is grown over the surface of the substrate (Inst. Phys. Conf. Ser. No. 79; Chapter 13, pp 737-738, 1985).
However, according to this method, the growth of a compound semiconductor is carried out by the thermal decomposition of the metallo-organic compound containing a Group III element and the hydride of a Group V element, so that it is impossible to lower the crystal growth temperature. In addition, the efficiency of raw materials or compounds to contribute to the growth of the compound semiconductor is low, and expensive and toxic gases must be used. Moreover, there arises the problem that impurities from the raw compounds are diffused into the grown crystal.